Methods and systems for determining a probe-off condition in a medical device

ABSTRACT

A physiological monitoring system may determine a probe-off condition. A physiological sensor may be used to emit one or more wavelengths of light. A received light signal may be processed to obtain a light signal corresponding to the emitted light and an ambient signal. The signals may be analyzed to identify similar behavior. The system may determine whether the physiological sensor is properly positioned based on the analysis.

The present disclosure relates to determining a sensor condition, andmore particularly relates to determining a probe-off condition in apulse oximeter or other medical device.

SUMMARY

Methods and systems are provided for determining whether a physiologicalsensor is properly positioned on a subject. In some embodiments, adetected light signal may be received using the physiological sensor.The detected light signal may be processed to obtain a first signalcorresponding to ambient light and a second signal corresponding to anemitted photonic signal and ambient light. The first and second signalmay be analyzed to determine similar behavior, and it may be determinedthat the physiological sensor is not properly positioned based on theanalysis.

In some embodiments, similar behaviors may include mimicking-equalbehavior, mimicking-parallel behavior, nonlinear scaling, any othersuitable behavior, or any combination thereof. For example,mimicking-equal or mimicking-parallel behavior may be identified whenboth the first and second signals move together or with a constantoffset. In some embodiments, this behavior may be identified as aprobe-off condition, and may be indicative of the same amount of lightreaching the detector irrespective of light emitted by the system. Insome embodiments, the system may identify a constant amplitude or signalflatness. A constant amplitude may be used alone or in combination withother signals to determine a probe-off condition.

BRIEF DESCRIPTION OF THE FIGURES

The above and other features of the present disclosure, its nature andvarious advantages will be more apparent upon consideration of thefollowing detailed description, taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a block diagram of an illustrative physiological monitoringsystem in accordance with some embodiments of the present disclosure;

FIG. 2A shows an illustrative plot of a light drive signal including ared drive pulse and an IR drive pulse in accordance with someembodiments of the present disclosure;

FIG. 2B shows an illustrative plot of a detector current waveform thatmay be generated by a sensor in accordance with some embodiments of thepresent disclosure;

FIG. 3 is a perspective view of an embodiment of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 4 shows an illustrative signal processing system in accordance withsome embodiments of the present disclosure;

FIG. 5 is a flow diagram showing illustrative steps for determiningwhether a sensor is properly positioned in accordance with someembodiments of the present disclosure;

FIG. 6 shows illustrative plots of physiological monitoring systemsignals in accordance with some embodiments of the present disclosure;

FIG. 7 shows further illustrative plots of physiological monitoringsystem signals in accordance with some embodiments of the presentdisclosure;

FIG. 8 shows a further illustrative plot of physiological monitoringsystem signals in accordance with some embodiments of the presentdisclosure;

FIG. 9 is a flow diagram showing illustrative steps for determining aprobe-off condition in accordance with some embodiments of the presentdisclosure; and

FIG. 10 shows an illustrative plot of a physiological monitoring systemsignal in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE FIGURES

The present disclosure is directed towards determining a probe-offcondition in a medical device. A physiological monitoring system maymonitor one or more physiological parameters of a patient, typicallyusing one or more physiological sensors. For example, the physiologicalmonitoring system may include a pulse oximeter. The system may include,for example, a light source and a photosensitive detector. In someembodiments, a sensor may be attached to a target area of a patient. Forexample, the sensor may be attached using an adhesive, a strap, a band,elastic, any other suitable attachment, or any combination thereof. Insome embodiments, the sensor may be located proximate to a desiredstructural element. For example, a sensor may be held near to the radialartery using a wrist strap. In another example, a sensor may be heldnear to the blood vessels of the forehead using an adhesive, tape, aheadband strap, any other suitable attachment, or any combinationthereof. In a further example, a sensor may be held near the bloodvessels on a fingertip using an adhesive, tape or mechanical clip.

In some embodiments, the system may determine a probe-off condition. Asused herein, the probe-off condition may include any condition where thesensor is fully or partially detached or moved from the desired targetarea of the subject. A probe-off condition may include a condition wherean adhesive coupling the sensor to the subject has fully or partiallyfailed. A probe-off condition may include a condition where a sensorheld with a strap or band has loosened, shifted, slid, moved, detached,repositioned in any other unsuitable arrangement, or any combinationthereof. For example, a sensor held by an adhesive to the forehead of asubject may fully or partially separate due to an adhesive failure,resulting in a probe-off condition. In another example, a sensor heldproximal to the radial artery at the wrist of a subject by a strap orband may shift out of position, resulting in a probe-off position. Itwill be understood that the probe-off conditions described here aremerely exemplary and that any suitable undesirable positioning of thesensor may result in a probe-off condition. It will also be understoodthat the particular arrangement of a probe-off condition may dependentupon the configuration and type of probe.

The probe-off condition may be detected by the system. In someembodiments, the system may use a detected ambient signal and a detectedlight signal to determine a probe-off condition. As used herein, theemitted photonic signal is a light signal emitted by the system. As usedherein, a received light signal is a light signal received by a detectorof the system. As used herein, a detected light signal is a light signalcorresponding to light detected during the “on” period of a multiplexeddrive. As used herein, a detected emitted light signal is a light signalcorresponding to light from the emitted photonic signal. As used herein,a detected ambient light signal is a light signal corresponding to lightdetected during the “off” period of a multiplexed drive signal. In someembodiments, the received light signal is split into one or moredetected light signals and one or more ambient light signals bydemultiplexing. It will be understood that in some embodiments, thedetected light signal may be the same or similar to the detected emittedlight signal (e.g., when there is no ambient light or when the ambientlight is filtered out prior to detection).

As will be described in detail below, a detected ambient signal mayinclude information related to the amount of light a detector receiveswhen one or more associated light sources are in an “off” state.Detected ambient signals may be time division multiplexed in a drivepulse modulation technique. In some embodiments where a detectorreceives light from light sources coupled to the system and from lightsources not coupled to the system, the detected ambient signal mayinclude the light from the light sources not coupled to the system.Ambient light sources may include sunlight, incandescent room lights,fluorescent room lights, fireplaces, candles, naked flames, LED roomlights, instrument panel lighting, heat sources, any other suitablelight sources not intended for determining a physiological parameter, orany combination thereof. It will be understood that heat sources maygenerate non-visible IR light that may be detected by the system. Itwill be understood that any visible or non-visible source ofelectromagnetic radiation may be included in the detected ambient signalincluding, for example, radio waves, microwave, IR, visible, UV, X-ray,gamma ray. In some arrangements, the detected ambient signal may includedecaying LED light from the system light sources. For example, it maytake a particular amount of time for the light output from a lightsource to decrease to zero following the light drive signal beingswitched off. A portion of this emitted light may be included in thedetected ambient signal. In some arrangements, the detected ambientsignal may not contain physiological information.

As will be described in detail below, a detected light signal mayinclude information related to the amount of light a detector receiveswhen one or more associated light sources are in an “on” state. In someembodiments where a detector receives light from light sources coupledto the system and from light sources not coupled to the system, thedetected light signal may include light from both sources. Detectedlight signals may be time division demultiplexed in a drive pulsemodulation or other suitable technique. As described above, a detectedlight signal may include both a detected emitted photonic signalcomponent and an ambient signal component. In some embodiments, anambient signal may be subtracted or otherwise separated from a detectedlight signal. In some embodiments, a detected light signal baseline maybe identified based on the detected light signal and the detectedambient signal.

In some embodiments, a sensor may be configured to limit the amount ofambient light received by a detector. For example, a detector may beheld close to and facing the skin. A detector may include a lightblocking material between the detector and any ambient light sources, toprevent or reduce ambient light from reaching the detector. In a furtherexample, a system may include other suitable shields, optics, filters,arrangements, or any combination thereof, to reduce ambient lightsignals received by the receiver. In some embodiments, the particulararrangement of light blocking structures or material may depend on thetype of probe. For example, a forehead probe may include a relativelyflat light blocking structure, while a fingertip probe may include alight blocking structure that encircles the finger. It will beunderstood, however, that may clinical settings include relativelybright light sources and the ambient light signals received by thedetector may not be fully blocked when the sensor is positioned asdesired. Similarly, fully shielding ambient light may be more difficultfor a forehead sensor than, for example, a fingertip sensor.

In some embodiments, for example, with a fingertip sensor where lightmay be generated by the system on one side of a finger and detected onthe opposite side of a finger, removing the finger from the sensor(i.e., a probe-off condition) may result in a large portion of thegenerated light being received by the sensor, rather than a portion ofthe light being attenuated by interacting with the tissue of thesubject. This very high signal level may be detected as a probe-offcondition by the system.

In some embodiments, for example, with a forehead sensor, a probe-offcondition may not result in a relatively high detected signal level. Aforehead sensor may include a light source placed relatively close to adetector on the forehead of a patient using tape, an adhesive, a bandencircling the skull, any other suitable arrangement, or any combinationthereof. The light source and detector may be arranged such that aportion of the light emitted from the light source interacts with, andis partially attenuated by, the tissue of the subject and is detected bythe detector. The light source may be pulsed, such that a detectedambient signal is detected by the detector between the pulses, and adetected light signal is detected during the pulses including both adetected ambient light signal and a detected emitted photonic signal. Indetermining a physiological parameter, the detected ambient light signalmay be, for example, subtracted from the detected light signal. Thedetected ambient signal may exhibit certain characteristic behaviorduring a probe-off condition, but may remain relatively constant withrespect to other changes. For example, during a probe-off condition, thedetected ambient light signal may be relatively insensitive to changesin physiological conditions.

In some embodiments, the system may analyze a first signal and a secondsignal to identify similar behavior. For example, a level or trend ofthe detected ambient signal may be compared to a level or trend of adetected light signal (e.g., an IR signal). In some embodiments, thedetected light signal may include light from a red light emitting diode,an infrared light emitting diode, ambient light, any other suitablelight source, or any combination thereof.

In some embodiments, a detected light signal (e.g., an IR signal) and adetected ambient signal may have similar or equal amplitudes. This maybe referred to as mimicking-equal behavior. In some embodiments, thesystem may identify mimicking-equal behavior as being indicative of aprobe-off condition. This may occur, for example, when only ambientlight is reaching the detector. In this case, the detected ambient anddetected light signals are of the same level because they bothcorrespond to the ambient light level. That is to say, the same amountof light is reaching the detector during the “on” and “off” states ofthe emitters. Mimicking-equal behavior may be identified, for example,when the subtracted difference of one signal from the other is constantwithin a particular tolerance.

In some embodiments, a detected light signal (e.g., an IR signal) and adetected ambient signal may show similar or parallel variations,referred to as mimicking-parallel behavior. This may be identified, forexample, when a first signal is subtracted from a second signal and theresult is substantially constant. The difference may be evaluated by itsderivative, by changes within a time window, by any other suitabletechnique, or any combination thereof. In some embodiments, the systemmay identify mimicking-parallel behavior as being indicative of aprobe-off condition.

In some embodiments, two signals may vary in a substantially similarmanner but the amplitude of a first signal may change more significantlythan the amplitude of a second. For example, this behavior may beexemplified where a first signal subtracted from the product of a secondsignal and an appropriate scaling factor is equal to zero. This may bereferred to as nonlinear scaling. Nonlinear scaling may occur when, forexample, mimicking-parallel or mimicking-equal behavior might occur, butone signal displays relatively higher amplitude variations as a resultof emitter nonlinearity, detector nonlinearity, gain nonlinearity,processing nonlinearity, other suitable signal variations, or anycombination thereof. In some embodiments, the system may identifymimicking with nonlinear scaling as being indicative of a probe-offcondition.

In some embodiments, a signal may include a substantially constantamplitude. The substantially constant amplitude may, for example, beidentified by the derivative of a signal within a time window. Thissubstantially constant amplitude may be referred to as flatness. In someembodiments, the system may identify flatness as being indicative of aprobe-off condition. For example, a detected light signal from aproperly positioned probe may normally fluctuate with patientphysiological parameters, and thus flatness may indicate that the probeis not properly positioned. Flatness may be identified in a detectedlight signal, a detected ambient signal, any other suitable signal, orany combination thereof.

It will be understood that any of the aforementioned signal behaviorsmay be used in any suitable combination. For example, a change in thelevel of the ambient light (e.g., an examination light is switched on)may be distinguished from a probe-off condition by comparing multiplesignal levels, by a trend, by a rate of change, by user input, by anyother suitable technique, or any combination thereof. It will also beunderstood that the aforementioned signal behaviors are merely exemplaryand that any suitable signal characteristics or combinations ofcharacteristics may be used to identify a probe-off condition. Infurther examples, probe-off condition characteristics may include strongambient light, weak detected light signals, and boundary phase offsetconditions.

The foregoing probe-off techniques may be implemented in an oximeter. Anoximeter is a medical device that may determine the oxygen saturation ofan analyzed tissue. One common type of oximeter is a pulse oximeter,which may non-invasively measure the oxygen saturation of a patient'sblood (as opposed to measuring oxygen saturation directly by analyzing ablood sample taken from the patient). Pulse oximeters may be included inpatient monitoring systems that measure and display various blood flowcharacteristics including, but not limited to, the oxygen saturation ofhemoglobin in arterial blood. Such patient monitoring systems may alsomeasure and display additional physiological parameters, such as apatient's pulse rate and blood pressure.

An oximeter may include a light sensor that is placed at a site on apatient, typically a fingertip, toe, forehead or earlobe, or in the caseof a neonate, across a foot or hand. The oximeter may use a light sourceto pass light through blood perfused tissue and photoelectrically sensethe absorption of the light in the tissue. In addition, locations whichare not typically understood to be optimal for pulse oximetry serve assuitable sensor locations for the blood pressure monitoring processesdescribed herein, including any location on the body that has a strongpulsatile arterial flow. For example, additional suitable sensorlocations include, without limitation, the neck to monitor carotidartery pulsatile flow, the wrist to monitor radial artery pulsatileflow, the inside of a patient's thigh to monitor femoral arterypulsatile flow, the ankle to monitor tibial artery pulsatile flow, andaround or in front of the ear. Suitable sensors for these locations mayinclude sensors for sensing absorbed light based on detecting reflectedlight. In all suitable locations, for example, the oximeter may measurethe intensity of light that is received at the light sensor as afunction of time. The oximeter may also include sensors at multiplelocations. A signal representing light intensity versus time or amathematical manipulation of this signal (e.g., a scaled versionthereof, a log taken thereof, a scaled version of a log taken thereof,etc.) may be referred to as the photoplethysmograph (PPG) signal. Inaddition, the term “PPG signal,” as used herein, may also refer to anabsorption signal (i.e., representing the amount of light absorbed bythe tissue) or any suitable mathematical manipulation thereof. The lightintensity or the amount of light absorbed may then be used to calculateany of a number of physiological parameters, including an amount of ablood constituent (e.g., oxyhemoglobin) being measured as well as apulse rate and when each individual pulse occurs.

In some embodiments, the photonic signal interacting with the tissue isselected to be of one or more wavelengths that are attenuated by theblood in an amount representative of the blood constituentconcentration. Red and infrared (IR) wavelengths may be used because ithas been observed that highly oxygenated blood will absorb relativelyless red light and more IR light than blood with a lower oxygensaturation. By comparing the intensities of two wavelengths at differentpoints in the pulse cycle, it is possible to estimate the blood oxygensaturation of hemoglobin in arterial blood.

The system may process data to determine physiological parameters usingtechniques well known in the art. For example, the system may determineblood oxygen saturation using two wavelengths of light and aratio-of-ratios calculation. The system also may identify pulses anddetermine pulse amplitude, respiration, blood pressure, other suitableparameters, or any combination thereof, using any suitable calculationtechniques. In some embodiments, the system may use information fromexternal sources (e.g., tabulated data, secondary sensor devices) todetermine physiological parameters.

In some embodiments, a light drive modulation may be used. For example,a first light source may be turned on for a first drive pulse, followedby an off period, followed by a second light source for a second drivepulse, followed by an off period. The first and second drive pulses maybe used to determine physiological parameters. The off periods may beused to determine detected ambient signal levels, reduce overlap of thelight drive pulses, allow time for light sources to stabilize, allowtime for detected light signals to stabilize or settle, reduce heatingeffects, reduce power consumption, for any other suitable reason, or anycombination thereof.

It will be understood that the probe-off techniques described herein arenot limited to pulse oximeters and may be applied to any suitablemedical and non-medical devices. For example, the system may includeprobes for regional saturation (rSO2), respiration rate, respirationeffort, continuous non-invasive blood pressure, saturation patterndetection, fluid responsiveness, cardiac output, any other suitableclinical parameter, or any combination thereof. Probes may be used witha pulse oximeter, a general purpose medical monitor, any other suitablemedical device, or any combination thereof. In some embodiments, theprobe-off identification techniques described herein may be applied toanalysis of light levels where an ambient or dark signal is detected.

The following description and accompanying FIGS. 1-10 provide additionaldetails and features of some embodiments of determining a sensorcondition in a medical device.

FIG. 1 is a block diagram of an illustrative physiological monitoringsystem 100 in accordance with some embodiments of the presentdisclosure. System 100 may include a sensor 102 and a monitor 104 forgenerating and processing physiological signals of a subject. In someembodiments, sensor 102 and monitor 104 may be part of an oximeter.

Sensor 102 of physiological monitoring system 100 may include lightsource 130 and detector 140. Light source 130 may be configured to emitphotonic signals having one or more wavelengths of light (e.g. Red andIR) into a subject's tissue. For example, light source 130 may include aRed light emitting light source and an IR light emitting light source,e.g. Red and IR light emitting diodes (LEDs), for emitting light intothe tissue of a subject to generate physiological signals. In oneembodiment, the Red wavelength may be between about 600 nm and about 700nm, and the IR wavelength may be between about 800 nm and about 1000 nm.It will be understood that light source 130 may include any number oflight sources with any suitable characteristics. In embodiments where anarray of sensors is used in place of single sensor 102, each sensor maybe configured to emit a single wavelength. For example, a first sensormay emit only a Red light while a second may emit only an IR light.

It will be understood that, as used herein, the term “light” may referto energy produced by radiative sources and may include one or more ofultrasound, radio, microwave, millimeter wave, infrared, visible,ultraviolet, gamma ray or X-ray electromagnetic radiation. As usedherein, light may also include any wavelength within the radio,microwave, infrared, visible, ultraviolet, or X-ray spectra, and thatany suitable wavelength of electromagnetic radiation may be appropriatefor use with the present techniques. Detector 140 may be chosen to bespecifically sensitive to the chosen targeted energy spectrum of lightsource 130.

In some embodiments, detector 140 may be configured to detect theintensity of light at the Red and IR wavelengths. In some embodiments,an array of sensors may be used and each sensor in the array may beconfigured to detect an intensity of a single wavelength. In operation,light may enter detector 140 after passing through the subject's tissue.Detector 140 may convert the intensity of the received light into anelectrical signal. The light intensity may be directly related to theabsorbance and/or reflectance of light in the tissue. That is, when morelight at a certain wavelength is absorbed, scattered, or reflected, lesslight of that wavelength is typically received from the tissue bydetector 140. After converting the received light to an electricalsignal, detector 140 may send the detection signal to monitor 104, wherethe detection signal may be processed and physiological parameters maybe determined (e.g., based on the absorption of the Red and IRwavelengths in the subject's tissue). In some embodiments, the detectionsignal may be preprocessed by sensor 102 before being transmitted tomonitor 104.

In the embodiment shown, monitor 104 includes control circuitry 110,light drive circuitry 120, front end processing circuitry 150, back endprocessing circuitry 170, user interface 180, and communicationinterface 190. Monitor 104 may be communicatively coupled to sensor 102.

Control circuitry 110 may be coupled to light drive circuitry 120, frontend processing circuitry 150, and back end processing circuitry 170, andmay be configured to control the operation of these components. In someembodiments, control circuitry 110 may be configured to provide timingcontrol signals to coordinate their operation. For example, light drivecircuitry 120 may generate a light drive signal, which may be used toturn on and off the light source 130, based on the timing controlsignals. The front end processing circuitry 150 may use the timingcontrol signals of control circuitry 110 to operate synchronously withlight drive circuitry 120. For example, front end processing circuitry150 may synchronize the operation of an analog-to-digital converter anda demultiplexer with the light drive signal based on the timing controlsignals. In addition, the back end processing circuitry 170 may use thetiming control signals of control circuitry 110 to coordinate itsoperation with front end processing circuitry 150.

Light drive circuitry 120, as discussed above, may be configured togenerate a light drive signal that is provided to light source 130 ofsensor 102. The light drive signal may, for example, control theintensity of light source 130 and the timing of switching light source130 on and off. When light source 130 is configured to emit two or morewavelengths of light, the light drive signal may be configured tocontrol the operation of each wavelength of light. The light drivesignal may comprise a single signal or may comprise multiple signals(e.g., one signal for each wavelength of light). An illustrative lightdrive signal is shown in FIG. 2A.

FIG. 2A shows an illustrative plot of a light drive signal including reddrive pulse 202 and IR drive pulse 204 in accordance with someembodiments of the present disclosure. Red drive pulse 202 and IR drivepulse 204 may be generated by light drive circuitry 120 under thecontrol of control circuitry 110. As used herein, drive pulses may referto switching power or other components on and off, high and low outputstates, high and low values within a continuous modulation, othersuitable relatively distinct states, or any combination thereof. Thelight drive signal may be provided to light source 130, including reddrive pulse 202 and IR drive pulse 204 to drive red and IR lightemitters, respectively, within light source 130. Red drive pulse 202 mayhave a higher amplitude than IR drive pulse 204 since red LEDs may beless efficient than IR LEDs at converting electrical energy into lightenergy. In some embodiments, the output levels may be the equal, may beadjusted for nonlinearity of emitters, may be modulated in any othersuitable technique, or any combination thereof. Additionally, red lightmay be absorbed and scattered more than IR light when passing throughperfused tissue. When the red and IR light sources are driven in thismanner they emit pulses of light at their respective wavelengths intothe tissue of a subject in order generate physiological signals thatphysiological monitoring system 100 may process to calculatephysiological parameters. It will be understood that the light driveamplitudes of FIG. 2A are merely exemplary any that any suitableamplitudes or combination of amplitudes may be used, and may be based onthe light sources, the subject tissue, the determined physiologicalparameter, modulation techniques, power sources, any other suitablecriteria, or any combination thereof.

The light drive signal of FIG. 2A may also include “off” periods 220between the Red and IR light drive pulse. “Off” periods 220 are periodsduring which no drive current may be applied to light source 130. “Off”periods 220 may be provided, for example, to prevent overlap of theemitted light, since light source 130 may require time to turncompletely on and completely off. Similarly, the signal from detector140 may require time to decay completely to a final state after lightsource 130 is switched off. The period from time 216 to time 218 may bereferred to as a drive cycle, which includes four segments: a red drivepulse 202, followed by an “off” period 220 in FIG. 2A, followed by an IRdrive pulse 204, and followed by an “off” period 220. After time 218,the drive cycle may be repeated (e.g., as long as a light drive signalis provided to light source 130). It will be understood that thestarting point of the drive cycle is merely illustrative and that thedrive cycle can start at any location within FIG. 2A, provided the cyclespans two drive pulses and two “off” periods. Thus, each red drive pulse202 and each IR drive pulse 204 may be understood to be surrounded bytwo “off” periods 220 in FIG. 2A. “Off” periods may also be referred toas dark periods, in that the emitters are dark or returning to darkduring that period.

Referring back to FIG. 1, front end processing circuitry 150 may receivea detection signal from detector 140 and provide one or more processedsignals to back end processing circuitry 170. The term “detectionsignal,” as used herein, may refer to any of the signals generatedwithin front end processing circuitry 150 as it processes the outputsignal of detector 140. Front end processing circuitry 150 may performvarious analog and digital processing of the detector signal. Onesuitable detector signal that may be received by front end processingcircuitry 150 is shown in FIG. 2B.

FIG. 2B shows an illustrative plot of detector current waveform 214 thatmay be generated by a sensor in accordance with some embodiments of thepresent disclosure. The peaks of detector current waveform 214 mayrepresent current signals provided by a detector, such as detector 140of FIG. 1, when light is being emitted from a light source. Theamplitude of detector current waveform 214 may be proportional to thelight incident upon the detector. The peaks of detector current waveform214 may be synchronous with drive pulses driving one or more emitters ofa light source, such as light source 130 of FIG. 1. For example,detector current waveform 214 may be generated in response to a lightsource being driven by the light drive signal of FIG. 2A. The valleys ofdetector current waveform 214 may be synchronous with periods of timeduring which no light is being emitted by the light source. While nolight is being emitted by a light source during the valleys, detectorcurrent waveform 214 may not decrease to zero. Rather, ambient signal222 may be present in the detector waveform, as well as other backgroundamplitude contributions. In some embodiments, the shapes of the pulsesin detector current waveforms may include distortions, clipping, delayedswitching, relatively slow rising and falling edges, and othernon-idealities due in part to switching of the light drive, switching ofthe LEDs, switching in the detector, stabilization of the detector,stabilization of the detected signal, parasitic inductance and/orcapacitances, any other suitable signal contribution, or any combinationthereof. In some embodiments, ambient signal 222 may be used todetermine a probe-off condition. In some embodiments, ambient signal 222may be removed from a processed signal to facilitate determination ofphysiological parameters.

Referring back to FIG. 1, front end processing circuitry 150, which mayreceive a detection signal, such as detector current waveform 214, mayinclude analog conditioner 152, demultiplexer 154, digital conditioner156, analog-to-digital converter (ADC) 158, decimator/interpolator 160,and ambient subtractor 162.

In some embodiments, front end processing circuitry 150 may include asecond analog-to-digital converter (not shown) configured to sample theunprocessed detector signal. This signal may be used to detect changesin the ambient light level without applying the signal condition andother steps that may improve the quality of determined physiologicalparameters but may reduce the amount of information regarding aprobe-off condition.

Analog conditioner 152 may perform any suitable analog conditioning ofthe detector signal. The conditioning performed may include any type offiltering (e.g., low pass, high pass, band pass, notch, or any othersuitable filtering), amplifying, performing an operation on the receivedsignal (e.g., taking a derivative, averaging), performing any othersuitable signal conditioning (e.g., converting a current signal to avoltage signal), or any combination thereof.

The conditioned analog signal may be processed by analog-to-digitalconverter 158, which may convert the conditioned analog signal into adigital signal. Analog-to-digital converter 158 may operate under thecontrol of control circuitry 110. Analog-to-digital converter 158 mayuse timing control signals from control circuitry 110 to determine whento sample the analog signal. Analog-to-digital converter 158 may be anysuitable type of analog-to-digital converter of sufficient resolution toenable a physiological monitor to accurately determine physiologicalparameters.

Demultiplexer 154 may operate on the analog or digital form of thedetector signal to separate out different components of the signal. Forexample, detector current waveform 214 of FIG. 2B includes a Redcomponent, an IR component, and at least one ambient component.Demultiplexer 154 may operate on detector current waveform 214 of FIG.2B to generate a Red signal, an IR signal, a first ambient signal (e.g.,corresponding to the ambient component that occurs immediately after theRed component), and a second ambient signal (e.g., corresponding to theambient component that occurs immediately after the IR component).Demultiplexer 154 may operate under the control of control circuitry110. For example, demultiplexer 154 may use timing control signals fromcontrol circuitry 110 to identify and separate out the differentcomponents of the detector signal.

Digital conditioner 156 may perform any suitable digital conditioning ofthe detector signal. Digital conditioner 156 may include any type ofdigital filtering of the signal (e.g., low pass, high pass, band pass,notch, or any other suitable filtering), amplifying, performing anoperation on the signal, performing any other suitable digitalconditioning, or any combination thereof.

Decimator/interpolator 160 may decrease the number of samples in thedigital detector signal. For example, decimator/interpolator 160 maydecrease the number of samples by removing samples from the detectorsignal or replacing samples with a smaller number of samples. Thedecimation or interpolation operation may include or be followed byfiltering to smooth the output signal.

Ambient subtractor 162 may operate on the digital signal. In someembodiments, ambient subtractor 162 may remove ambient values from theRed and IR components. In some embodiments, the system may subtract theambient values from the Red and IR components to generate adjusted Redand IR signals. For example, ambient subtractor 162 may determine asubtraction amount from the ambient signal portion of the detectionsignal and subtract it from the peak portion of the detection signal inorder to reduce the effect of the ambient signal on the peak. Forexample, in reference to FIG. 2A, a detection signal peak correspondingto red drive pulse 202 may be adjusted by determining the amount ofambient signal during the “off” period 220 preceding red drive pulse202. The ambient signal amount determined in this manner may besubtracted from the detector peak corresponding to red drive pulse 202.Alternatively, the “off” period 220 after red drive pulse 202 may beused to correct red drive pulse 202 rather than the “off” period 220preceding it. Additionally, an average of the “off” periods 220 beforeand after “on” periods of red drive pulse 202 may be used. In someembodiments, ambient subtractor 162 may output an ambient signal forfurther processing. Ambient subtractor 162 may average the ambientsignal from multiple “off” periods 220, may apply filters to the ambientsignal such as averaging filters, integration filters, delay filters,buffers, counters, any other suitable filters or processing, or anycombination thereof.

It will be understood that in some embodiments, ambient subtractor 162may be omitted. It will also be understood that in some embodiments, thesystem may not subtract the ambient contribution of the signal. It willalso be understood that the functions of demultiplexer 154 and ambientsubtractor 162 may be complementary, overlapping, combined into a signalfunction, combined or separated in any suitable arrangement, or anycombination thereof. For example, the received light signal may includean ambient signal, an IR light signal, and a red light signal. Thesystem may use any suitable arrangement of demultiplexer 154 and ambientsubtractor 162 to determine or generate any combination of: a redsignal, an IR signal, a red ambient signal, an IR ambient signal, anaverage ambient signal, a red with ambient signal, an IR with ambientsignal, any other suitable signal, or any combination thereof.

The components of front end processing circuitry 150 are merelyillustrative and any suitable components and combinations of componentsmay be used to perform the front end processing operations.

The front end processing circuitry 150 may be configured to takeadvantage of the full dynamic range of analog-to-digital converter 158.This may be achieved by applying a gain to the detected signal usinganalog conditioner 152 to map the expected range of the detection signalto the full or close to full dynamic range of analog-to-digitalconverter 158. In some embodiments, the input to the analog to digitalconverter may be the sum of the detected light multiplied by an analoggain value.

Ideally, when ambient light is zero and when the light source is off,the analog-to-digital converter 158 will read just above the minimuminput value. When the light source is on, the total analog gain may beset such that the output of analog-to-digital converter 158 may readclose to the full scale of analog-to-digital converter 158 withoutsaturating. This may allow the full dynamic range of analog-to-digitalconverter 158 to be used for representing the detection signal, therebyincreasing the resolution of the converted signal. In some embodiments,the total analog gain may be reduced by a small amount so that smallchanges in the light level incident on the detector do not causesaturation of analog-to-digital converter 158.

Back end processing circuitry 170 may include processor 172 and memory174. Processor 172 may be adapted to execute software, which may includean operating system and one or more applications, as part of performingthe functions described herein. Processor 172 may receive and furtherprocess physiological signals received from front end processingcircuitry 150. For example, processor 172 may determine one or morephysiological parameters based on the received physiological signals.Memory 174 may include any suitable computer-readable media capable ofstoring information that can be interpreted by processor 172. Thisinformation may be data or may take the form of computer-executableinstructions, such as software applications, that cause themicroprocessor to perform certain functions and/or computer-implementedmethods. Depending on the embodiment, such non-transitorycomputer-readable media may include computer storage media andcommunication media. Computer storage media may include volatile andnon-volatile, removable and non-removable media implemented in anymethod or technology for storage of information such ascomputer-readable instructions, data structures, program modules orother data. Computer storage media may include, but is not limited to,RAM, ROM, EPROM, EEPROM, flash memory or other solid state memorytechnology, CD-ROM, DVD, or other optical storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can be accessed by components of the system. Back endprocessing circuitry 170 may be communicatively coupled with userinterface 180 and communication interface 190.

User interface 180 may include user input 182, display 184, and speaker186. User input 182 may include any type of user input device such as akeyboard, a mouse, a touch screen, buttons, switches, a microphone, ajoy stick, a touch pad, or any other suitable input device. The inputsreceived by user input 182 can include information about the subject,such as age, weight, height, diagnosis, medications, treatments, and soforth. In an embodiment, the subject may be a medical patient anddisplay 184 may exhibit a list of values which may generally apply tothe patient, such as, for example, age ranges or medication families,which the user may select using user inputs 182. Additionally, display184 may display, for example, an estimate of a subject's blood oxygensaturation generated by monitor 104 (referred to as an “SpO₂”measurement), pulse rate information, respiration rate information,blood pressure, sensor condition, any other parameters, and anycombination thereof. Display 184 may include any type of display such asa cathode ray tube display, a flat panel display such a liquid crystaldisplay or plasma display, or any other suitable display device. Speaker186 within user interface 180 may provide an audible sound that may beused in various embodiments, such as for example, sounding an audiblealarm in the event that a patient's physiological parameters are notwithin a predefined normal range.

Communication interface 190 may enable monitor 104 to exchangeinformation with external devices. Communication interface 190 mayinclude any suitable hardware, software, or both, which may allowmonitor 104 to communicate with electronic circuitry, a device, anetwork, a server or other workstations, a display, or any combinationthereof. Communication interface 190 may include one or more receivers,transmitters, transceivers, antennas, plug-in connectors, ports,communications buses, communications protocols, device identificationprotocols, any other suitable hardware or software, or any combinationthereof. Communication interface 190 may be configured to allow wiredcommunication (e.g., using USB, RS-232 or other standards), wirelesscommunication (e.g., using WiFi, IR, WiMax, BLUETOOTH, UWB, or otherstandards), or both. For example, communication interface 190 may beconfigured using a universal serial bus (USB) protocol (e.g., USB 2.0,USB 3.0), and may be configured to couple to other devices (e.g., remotememory devices storing templates) using a four-pin USB standard Type-Aconnector (e.g., plug and/or socket) and cable. In some embodiments,communication interface 190 may include an internal bus such as, forexample, one or more slots for insertion of expansion cards.

It will be understood that the components of physiological monitoringsystem 100 that are shown and described as separate components are shownand described as such for illustrative purposes only. In someembodiments the functionality of some of the components may be combinedin a single component. For example, the functionality of front endprocessing circuitry 150 and back end processing circuitry 170 may becombined in a single processor system. Additionally, in some embodimentsthe functionality of some of the components of monitor 104 shown anddescribed herein may be divided over multiple components. For example,some or all of the functionality of control circuitry 110 may beperformed in front end processing circuitry 150, in back end processingcircuitry 170, or both. In other embodiments, the functionality of oneor more of the components may be performed in a different order or maynot be required. In some embodiments, all of the components ofphysiological monitoring system 100 can be realized in processorcircuitry.

FIG. 3 is a perspective view of an embodiment of a physiologicalmonitoring system 310 in accordance with some embodiments of the presentdisclosure. In some embodiments, one or more components of physiologicalmonitoring system 310 may include one or more components ofphysiological monitoring system 100 of FIG. 1. Physiological monitoringsystem 310 may include sensor unit 312 and monitor 314. In someembodiments, sensor unit 312 may be part of an oximeter. Sensor unit 312may include one or more light source 316 for emitting light at one ormore wavelengths into a subject's tissue. One or more detectors 318 mayalso be provided in sensor unit 312 for detecting the light that isreflected by or has traveled through the subject's tissue. Any suitableconfiguration of light source 316 and detector 318 may be used. In anembodiment, sensor unit 312 may include multiple light sources anddetectors, which may be spaced apart. Physiological monitoring system310 may also include one or more additional sensor units (not shown)that may, for example, take the form of any of the embodiments describedherein with reference to sensor unit 312. An additional sensor unit maybe the same type of sensor unit as sensor unit 312, or a differentsensor unit type than sensor unit 312 (e.g., a photoacoustic sensor).Multiple sensor units may be capable of being positioned at two or moredifferent locations on a subject's body.

In some embodiments, sensor unit 312 may be connected to monitor 314 asshown. Sensor unit 312 may be powered by an internal power source, e.g.,a battery (not shown). Sensor unit 312 may draw power from monitor 314.In another embodiment, the sensor may be wirelessly connected to monitor314 (not shown). Monitor 314 may be configured to calculatephysiological parameters based at least in part on data relating tolight emission and detection received from one or more sensor units suchas sensor unit 312. For example, monitor 314 may be configured todetermine pulse rate, blood pressure, blood oxygen saturation (e.g.,arterial, venous, or both), hemoglobin concentration (e.g., oxygenated,deoxygenated, and/or total), any other suitable physiologicalparameters, or any combination thereof. In some embodiments,calculations may be performed on the sensor units or an intermediatedevice and the result of the calculations may be passed to monitor 314.Further, monitor 314 may include display 320 configured to display thephysiological parameters or other information about the system. In theembodiment shown, monitor 314 may also include a speaker 322 to providean audible sound that may be used in various other embodiments, such asfor example, sounding an audible alarm in the event that a subject'sphysiological parameters are not within a predefined normal range orwhen a sensor is not properly positioned. In some embodiments,physiological monitoring system 310 may include a stand-alone monitor incommunication with the monitor 314 via a cable or a wireless networklink. In some embodiments, monitor 314 may be implemented as display 184of FIG. 1.

In some embodiments, sensor unit 312 may be communicatively coupled tomonitor 314 via a cable 324. Cable 324 may include electronic conductors(e.g., wires for transmitting electronic signals from detector 318),optical fibers (e.g., multi-mode or single-mode fibers for transmittingemitted light from light source 316), any other suitable components, anysuitable insulation or sheathing, or any combination thereof. In someembodiments, a wireless transmission device (not shown) or the like maybe used instead of or in addition to cable 324. Monitor 314 may includea sensor interface configured to receive physiological signals fromsensor unit 312, provide signals and power to sensor unit 312, orotherwise communicate with sensor unit 312. The sensor interface mayinclude any suitable hardware, software, or both, which may be allowcommunication between monitor 314 and sensor unit 312.

In some embodiments, physiological monitoring system 310 may includecalibration device 380. Calibration device 380, which may be powered bymonitor 314, a battery, or by a conventional power source such as a walloutlet, may include any suitable calibration device. Calibration device380 may be communicatively coupled to monitor 314 via communicativecoupling 382, and/or may communicate wirelessly (not shown). In someembodiments, calibration device 380 is completely integrated withinmonitor 314. In some embodiments, calibration device 380 may include amanual input device (not shown) used by an operator to manually inputreference signal measurements obtained from some other source (e.g., anexternal invasive or non-invasive physiological measurement system).

In the illustrated embodiment, physiological monitoring system 310includes a multi-parameter physiological monitor 326. The display 328 ofmulti-parameter physiological monitor 326 may include a cathode ray tubedisplay, a flat panel display (as shown) such as a liquid crystaldisplay (LCD) or a plasma display, or may include any other type ofmonitor now known or later developed. Multi-parameter physiologicalmonitor 326 may be configured to calculate physiological parameters andto provide information from monitor 314 and from other medicalmonitoring devices or systems (not shown) using display 328. Forexample, multi-parameter physiological monitor 326 may be configured todisplay an estimate of a subject's blood oxygen saturation andhemoglobin concentration generated by monitor 314. Multi-parameterphysiological monitor 326 may include a speaker 330.

Monitor 314 may be communicatively coupled to multi-parameterphysiological monitor 326 via a cable 332 or 334 that is coupled to asensor input port or a digital communications port, respectively and/ormay communicate wirelessly (not shown). In addition, monitor 314 and/ormulti-parameter physiological monitor 326 may be coupled to a network toenable the sharing of information with servers or other workstations(not shown). Monitor 314 may be powered by a battery (not shown) or by aconventional power source such as a wall outlet.

In some embodiments, all or some of monitor 314 and multi-parameterphysiological monitor 326 may be referred to collectively as processingequipment.

FIG. 4 shows illustrative signal processing system 400 in accordancewith some embodiments of the present disclosure. Signal processingsystem 400 includes input signal generator 410, processor 412 and output414. In the illustrated embodiment, input signal generator 410 mayinclude pre-processor 420 coupled to sensor 418. As illustrated, inputsignal generator 410 generates an input signal 416. In some embodiments,input signal 416 may include one or more intensity signals based on adetector output. In some embodiments, pre-processor 420 may be anoximeter and input signal 416 may be a PPG signal. In an embodiment,pre-processor 420 may be any suitable signal processing device and inputsignal 416 may include PPG signals and one or more other physiologicalsignals, such as an electrocardiogram (ECG) signal. It will beunderstood that input signal generator 410 may include any suitablesignal source, signal generating data, signal generating equipment, orany combination thereof to produce input signal 416. Input signal 416may be a single signal, or may be multiple signals transmitted over asingle pathway or multiple pathways.

Pre-processor 420 may apply one or more signal processing operations tothe signal generated by sensor 418. For example, pre-processor 420 mayapply a pre-determined set of processing operations to the signalprovided by sensor 418 to produce input signal 416 that can beappropriately interpreted by processor 412, such as performing A/Dconversion. In some embodiments, A/D conversion may be performed byprocessor 412. Pre-processor 420 may also perform any of the followingoperations on the signal provided by sensor 418: reshaping the signalfor transmission, multiplexing the signal, modulating the signal ontocarrier signals, compressing the signal, encoding the signal, andfiltering the signal. In some embodiments, pre-processor 420 may includea current-to-voltage converter (e.g., to convert a photocurrent into avoltage), an amplifier, a filter, and A/D converter, a de-multiplexer,any other suitable pre-processing components, or any combinationthereof. In some embodiments, pre-processor 420 may include one or morecomponents from front end processing circuitry 150 of FIG. 1.

In some embodiments, input signal 416 may include PPG signalscorresponding to one or more light frequencies, such as an IR PPG signaland a Red PPG signal, and ambient light. In some embodiments, inputsignal 416 may include signals measured at one or more sites on asubject's body, for example, a subject's finger, toe, ear, arm, or anyother body site. In some embodiments, input signal 416 may includemultiple types of signals (e.g., one or more of an ECG signal, an EEGsignal, an acoustic signal, an optical signal, a signal representing ablood pressure, and a signal representing a heart rate). Input signal416 may be any suitable biosignal or any other suitable signal.

In some embodiments, input signal 416 may be coupled to processor 412.Processor 412 may be any suitable software, firmware, hardware, orcombination thereof for processing input signal 416. For example,processor 412 may include one or more hardware processors (e.g.,integrated circuits), one or more software modules, non-transitorycomputer-readable media such as memory, firmware, or any combinationthereof. Processor 412 may, for example, be a computer or may be one ormore chips (i.e., integrated circuits). Processor 412 may, for example,include an assembly of analog electronic components. Processor 412 maycalculate physiological information. For example, processor 412 maycompute one or more of a pulse rate, respiration rate, blood pressure,or any other suitable physiological parameter. Processor 412 may performany suitable signal processing of input signal 416 to filter inputsignal 416, such as any suitable band-pass filtering, adaptivefiltering, closed-loop filtering, any other suitable filtering, and/orany combination thereof. Processor 412 may also receive input signalsfrom additional sources (not shown). For example, processor 412 mayreceive an input signal containing information about treatments providedto the subject. Additional input signals may be used by processor 412 inany of the calculations or operations it performs in accordance withprocessing system 400.

In some embodiments, all or some of pre-processor 420, processor 412, orboth, may be referred to collectively as processing equipment.

Processor 412 may be coupled to one or more memory devices (not shown)or incorporate one or more memory devices such as any suitable volatilememory device (e.g., RAM, registers, etc.), non-volatile memory device(e.g., ROM, EPROM, magnetic storage device, optical storage device,flash memory, etc.), or both. The memory may be used by processor 412to, for example, store fiducial information or initializationinformation corresponding to physiological monitoring. In someembodiments, processor 412 may store physiological measurements orpreviously received data from input signal 416 in a memory device forlater retrieval. In some embodiments, processor 412 may store calculatedvalues, such as a pulse rate, a blood pressure, a blood oxygensaturation, a fiducial point location or characteristic, aninitialization parameter, or any other calculated values, in a memorydevice for later retrieval.

Processor 412 may be coupled to output 414. Output 414 may be anysuitable output device such as one or more medical devices (e.g., amedical monitor that displays various physiological parameters, amedical alarm, or any other suitable medical device that either displaysphysiological parameters or uses the output of processor 412 as aninput), one or more display devices (e.g., monitor, PDA, mobile phone,any other suitable display device, or any combination thereof), one ormore audio devices, one or more memory devices (e.g., hard disk drive,flash memory, RAM, optical disk, any other suitable memory device, orany combination thereof), one or more printing devices, any othersuitable output device, or any combination thereof.

It will be understood that system 400 may be incorporated intophysiological monitoring system 100 of FIG. 1. For example, input signalgenerator 410 may be implemented as part of sensor 102. In anotherexample, system 400 may be incorporated into physiological monitoringsystem 310 of FIG. 3, where input signal generator 410 may beimplemented as part of sensor unit 312 of FIG. 3. Processor 412 may beimplemented as part of monitor 104 of FIG. 1 or as part of monitor 314or multi-parameter physiological monitor 326 of FIG. 3. Output 414 maybe implemented as display 320 or 328 of FIG. 3. Furthermore, all or partof system 400 may be embedded in a small, compact object carried with orattached to the subject (e.g., a watch, other piece of jewelry, or asmart phone). In some embodiments, a wireless transceiver (not shown)may also be included in system 400 to enable wireless communication withother components of physiological monitoring systems 100 of FIG. 1 and310 of FIG. 3. As such, physiological monitoring systems 100 of FIG. 1and 310 of FIG. 3 may be part of a fully portable and continuous subjectmonitoring solution. In some embodiments, a wireless transceiver (notshown) may also be included in system 400 to enable wirelesscommunication with other components of physiological monitoring systems100 of FIG. 1 and 310 of FIG. 3. For example, pre-processor 420 maycommunicate input signal 416 over BLUETOOTH, 802.11, WiFi, WiMax, cable,satellite, Infrared, or any other suitable transmission scheme. In someembodiments, a wireless transmission scheme may be used between anycommunicating components of system 400. In some embodiments, system 400may include one or more communicatively coupled modules configured toperform particular tasks. In some embodiments, system 400 may beincluded as a module communicatively coupled to one or more othermodules.

It will be understood that the components of signal processing system400 that are shown and described as separate components are shown anddescribed as such for illustrative purposes only. In other embodimentsthe functionality of some of the components may be combined in a singlecomponent. For example, the functionality of processor 412 andpre-processor 420 may combined in a single processor system.Additionally, the functionality of some of the components shown anddescribed herein may be divided over multiple components. Additionally,signal processing system 400 may perform the functionality of othercomponents not show in FIG. 4. For example, some or all of thefunctionality of control circuitry 110 of FIG. 1 may be performed insignal processing system 400. In other embodiments, the functionality ofone or more of the components may not be required. In an embodiment, allof the components can be realized in processor circuitry.

In some embodiments, any of the processing components and/or circuits,or portions thereof, of FIGS. 1, 3, and 4 may be referred tocollectively as processing equipment. For example, processing equipmentmay be configured to amplify, filter, sample and digitize input signal416 (e.g., using an analog-to-digital converter), and calculatephysiological information from the digitized signal. Processingequipment may be configured to generate light drive signals, amplify,filter, sample and digitize detector signals, and calculatephysiological information from the digitized signal. In someembodiments, all or some of the components of the processing equipmentmay be referred to as a processing module.

FIG. 5 is flow diagram 500 showing illustrative steps for determiningwhether a sensor is properly positioned in accordance with someembodiments of the present disclosure.

In step 502, the system may use the physiological sensor to emit anemitted photonic signal. The system may emit the emitted photonic signalincluding one wavelength of light, multiple wavelengths of light, abroad spectrum light (e.g., white light), or any combination thereof.For example, the emitted photonic signal may include light from a redLED and light from an IR LED. The emitted photonic signal may beemitted, for example, by light source 130 of FIG. 1 or light source 316of FIG. 3. In some embodiments, the emitted photonic signal may includea light drive modulation. For example, when the emitted photonic signalincludes a red light source and an IR light source, the light drivemodulation may include a red drive pulse followed by an “off” periodfollowed by an IR drive pulse followed by an off period. It will beunderstood that this drive cycle modulation is merely exemplary and thatany suitable drive cycle modulation or combination of modulations may beused. In some embodiments, the emitted photonic signal may include acardiac cycle modulation, where the brightness, duty cycle, or otherparameters of one or more emitters are varied at a rate substantiallyrelated to the cardiac cycle.

In step 504, the system may receive a light signal. The received lightsignal may include the detected light signal and the detected ambientsignal, as described above. For example, the detected light signal andthe ambient signal may be determined by demultiplexing a received lightsignal based on time division multiplexing of the emitters. In someembodiments, the received light signal may in part correspond to theemitted photonic signals of step 502, light from ambient light sources,any other suitable source, or any combination thereof. The receivedlight signal may be received by, for example, detector 140 of FIG. 1 ordetector 318 of FIG. 3. In some embodiments, a portion of the emittedphotonic light signal may be partially attenuated by the tissue of thesubject before being detected as a received light signal. In someembodiments, the received light signal may have been primarily reflectedby the subject. For example, reflected light may be detected by aforehead-attached system where the emitter and detector are on the sameside of the subject. In some embodiments, the received light may be havebeen transmitted through the subject. For example, transmitted light maybe detected in a fingertip-attached or earlobe-attached sensor.

In some embodiments, the system may adjust or compensate a receivedlight signal depending in part on a light drive setting. Drive settingsmay include the LED drive signal, the detector gain, other suitablesystem parameters, or any combination thereof. For example, increasingthe gain on a detected signal may result in an increased level of thedetected ambient signal. The system may compensate for this increase inthe ambient light signal that is not correlated with a change in thesensor positioning, in part so that the gain change is distinguishedfrom a change in sensor positioning. In a further example, the systemmay change the LED emitter brightness, resulting in a change in thedetected light signal. The system may compensate for these changes inthe detected signal amplitude to distinguish them from a change in thesensor positioning. It will be understood that the system may make anyadjustments in gain, amplification, frequency, wavelength, amplitude,any other suitable adjustments, or any combination thereof. It will beunderstood that the adjustments may be made to the emitted photonicsignal, the received signal, any signal following a number of processingsteps, any other suitable signals, or any combination thereof.

In step 506, the system may process the received light signal to obtaina first signal. The first signal may correspond to detected ambientlight signal. In some embodiments, the detected ambient light signalmay, for example, correspond to detected ambient signal 222 of FIG. 2B.The first signal may be extracted from the received signal using, forexample, demultiplexer 154 of FIG. 1. The processing of the light signalto obtain the first signal may be implemented using any suitablecomponents of physiological monitoring system 100 of FIG. 1,physiological monitoring system 310 of FIG. 3, processing system 400 ofFIG. 4, any other suitable components, or any combination thereof.

In step 508, the system may process the received light signal to obtaina second signal. In some embodiments, the second signal may correspondto the detected light signal, that is, the combination of the detectedemitted photonic signal and the detected ambient light. In someembodiments, the second signal need not include the ambient lightcomponent. The second signal may correspond to one or more wavelengthsof emitted light, for example, an IR signal, a red signal, any othersuitable signal, or any combination thereof. The processing of the lightsignal to obtain the first signal may be implemented using any suitablecomponents of physiological monitoring system 100 of FIG. 1,physiological monitoring system 310 of FIG. 3, processing system 400 ofFIG. 4, any other suitable components, or any combination thereof.

In step 510, the system may analyze the first and second signals toidentify similar behavior. For example, the analysis may include acomparison of the first signal (e.g., the detected ambient light) to thesecond signal (e.g., the detected light signal). Comparisons may includesubtraction, division, multiplication, integration, any other suitablefunction, or any combination thereof. Comparisons may also includetime-domain comparisons. Similar behavior may include, for example,signals at equal amplitudes, signals showing parallel changes, signalswith the same or similar trends, signals with equal or similar slopes,signals changing in a non-linear but parallel trend, any other suitablesimilar behavior, or any combination thereof. For example, a detectedambient signal level may be compared to the moving average of the signalcorresponding to the emitted photonic signal. In some embodiments,comparing multiple signals may help identify a sensor-off or otherundesirable system condition from an external, unrelated change. Forexample, a large increase in ambient light caused by switching on anexamination room light source may cause the ambient light signal tocross a threshold, but, for example, may not cause the same change inthe detected emitted light signal. Thus, comparing the detected ambientsignal to another signal may help classify an ambient signal change. Inanother example, an external detector, for example, on the monitor, maybe used to determine an ambient light level that could be used tonormalize changes in the detected ambient signal of the sensor.

The system may determine characteristics of the first signal, the secondsignal, any other suitable signal, or any combination thereof.Characteristics may include the signal level, amplitude, rate of change,slope, moving average, other trend, any other suitable characteristic,or any combination thereof. For example, a trend may include a firstderivative of the amplitude signal. A characteristic may include acombination of parameters. For example, a trend may include themagnitude and polarity of the first derivative. In another example, thecharacteristic may include the signal amplitude and the polarity of thefirst derivative. Characteristics may be relative, absolute, or anycombination thereof. For example, the signal level may be the absoluteamplitude. In another example, the signal level may be relative to abaseline or to another signal. Determining the signal level may includeany suitable processing equipment described above. The system may applyto the light signal filtering, smoothing, averaging, any other suitabletechnique, or any combination thereof. For example, the light signal maybe filtered to remove noise. In another example, the signal may besmoothed or averaged to remove transient signals.

In some embodiments, the system may use complex and/or differentcomparisons of the two or more signals or characteristics of thesignals. For example, the system may compare the difference of theslopes of two signals to a threshold or target value to determineparallel behavior. The system may compare derivatives, averages,integrals, slopes, trends, any other suitable operation, or anycombination thereof. For example, the system may compare the firstderivatives of two signals by subtracting one from the other. In placeof subtraction, the system may also add, multiply, or divide. Morecomplex data sets may include matrix cross products, matrix dotproducts, convolutions, other suitable operations, or any combinationthereof. In some embodiments, signal processing may include wavelettransforms, scalograms, Fourier transforms, inverse Fourier transforms,other transforms, impulse response filters, any other suitable signalprocessing techniques, or any combination thereof.

In some embodiments, the system may include one or more target values orthreshold levels related to the comparison of signals or signalcharacteristics. The thresholds may be used to identify similar behaviorin multiple signals. A comparison metric reaching or crossing athreshold may result in an alarm being triggered, a flag being set, anindication being generated, a signal being generated, any other suitableoutput, or any combination thereof. For example, if the differencebetween two signals remains below 10 mV for 5 seconds, the behavior maybe identified as similar. Thresholds may be predetermined, set by theuser, determined based on historical information, determined based oncharacteristics related to the patient, determined based oncharacteristics of the sensor and system, determined based on any othersuitable criteria, or any combination thereof. Thresholds may beconstant or vary in time. The threshold may include multiple thresholdvalues corresponding to multiple characteristics.

In some embodiments, the threshold may be set during a reset period. Forexample, the reset period may be triggered by a user to indicate anormal operating state of the system. The normal operating state mayinclude proper positioning of the sensor. The reset mode may includesetting a normal level or trend for the detected ambient signal anddetermining a threshold based on that level or trend. In someembodiments, a reset period may be triggered automatically based ontime, sensor connections, signal conditions, a physiological conditionor event, any other suitable triggers, or any combination thereof.

In step 512, the system may determine that the physiological sensor isnot properly positioned. The system may determine this based on theanalysis of step 510. For example, if a particular similar behavior orrelationship is identified between the signals or signalcharacteristics, a probe-off condition may be identified. Similarbehaviors may include mimicking-equal behavior, mimicking-parallelbehavior, nonlinear scaling, any other suitable behavior, or anycombination thereof. In some embodiments, a behavior based on a singlecharacteristic (which may be derived from one or more signals) may beused to identify flatness, low signal level, high signal level, strongambient behavior, any other suitable behavior, or any combinationthereof. The system may identify the aforementioned behaviors as beingindicative of a probe-off condition.

In some embodiments, the system may use multiple criteria to determine aprobe-off condition. The multiple indicators may be combined using anysuitable logic technique, algorithmic technique, polling technique,weighted technique, any other suitable technique, or any combinationthereof. In some embodiments, a sequence of indicators may be includedin determining a probe-off condition. In some embodiments, the systemmay determine a confidence value related to the possibility of aprobe-off condition based on the criteria. In some embodiments, aconfidence value is compared to a threshold to identify a probe-offcondition.

In a subsequent step (not shown), the system may provide an indicatorthat the physiological sensor is not properly positioned. The indicationmay be based on any of the determinations that the sensor is improperlypositioned as described herein. Indicators may include audibleindications such as a voice alert, beep, or siren. Indicatorsadditionally or alternatively may include visual indications such aslights, LEDs, computer screen readouts, mechanical flags, blinking orpatterned use of any of the aforementioned, the use of particularcolors, any other suitable visual indicator, or any combination thereof.Indicators may additionally or alternatively include communications suchas communications sent to a cell phone, pager, text message, internetterminal, email, phone, fax, remote server, any other suitablecommunication, or any combination thereof. For example, a blinking redmessage may appear on a remote nurse's station computer in a hospitalsetting. In another example, a bedside monitor may emit a beeping soundwhen a probe-off condition is detected. In some embodiments, any or allof the aforementioned indicators may be graduated or otherwise alteredbased on a confidence that there is a probe-off condition, the durationof the probe-off condition, a sensitivity setting, any other suitableparameters, or any combination thereof.

It will be understood that the above described probe-off detectiontechniques are merely exemplary and that any suitable signalcharacteristics or combination of signal characteristics may be usedwith any suitable thresholds or combination of thresholds to determine aprobe-off condition. The following FIGS. 6-8 show illustrative examplesof detected ambient signals and detected light signals used to determinea probe-off condition.

FIG. 6 shows illustrative plots 640, 660, and 680 of physiologicalmonitoring system signals in accordance with some embodiments of thepresent disclosure. Plot 640 may include information related to adetected light signal, such as the light signal detected during the IR“on” period of a multiplexed received light signal. Plot 660 may includeinformation related to a detected ambient signal, such as the lightsignal detected during the “off” period of a multiplexed received lightsignal. Plot 680 may include information related to light levels andsensor positioning. The signals of plot 680 are indicative of manuallycontrolled test conditions, and the signals of plots 640 and 660 includesignals received under those test conditions. Plot 640, plot 660, andplot 680 may illustrate detected light signal and detected ambientsignal levels received by the system when the ambient light level andsensor position are varied. The signals of plots 640 and 660 andelsewhere may have been filtered, processed, scaled, or otherwisemodified such that typical pulse features are not illustrated in FIG. 6.For example, the periodic plethysmography waveforms may not be visibledue to the horizontal and or vertical scale of the plots. In anotherexample, the signals may be filtered by the system to remove thesevariations for the purposes of determining a probe-off condition. Itwill be understood that these signals and plots are merely exemplary andthat any suitable signals and analysis may be used.

Plot 640, plot 660, and plot 680 have abscissa axes in units of time. Insome embodiments, the axes may be on the same time scale. For example,the plots as illustrated may show approximately five minutes of signals.Plot 640 and plot 660 may have ordinate axes in units of amplitude. Plot680 may have arbitrary ordinate axes.

Plot 640 may include detected light signal 642. Detected light signal642 may include information related to an emitted photonic signalemitted by the system and detected ambient light. In some embodiments,detected light signal 642 need not include detected ambient light.Detected light signal 642 may have been processed by ambient subtractor162 of FIG. 1, demultiplexer 154 of FIG. 1, pre-processor 420 of FIG. 4,any other suitable processing equipment, or any combination thereof.Detected light signal 642 may include information related to lightdetected during drive pulses of a drive pulse modulation, such as reddrive pulse 202 or IR drive pulse 204 of FIG. 2A. For example, detectedlight signal 642 may include information related to the amplitude of thereceived IR signal. In some embodiments, some or all of the IR lightemitted by the system may be attenuated, reflected, absorbed, orotherwise altered by interacting with the subject. In some embodiments,detected light signal 642 may include information related to theamplitude of the received red signal. In some embodiments, detectedlight signal 642 may include information related to any suitable lightsource, or any combination of light sources. In some embodiments,information from light sources may be combined to generated detectedlight signal 642 by averaging, summation, weighted summation,integration, any other suitable processing step, or any combinationthereof. For example, information from a red and IR signal may benormalized and combined to generate detected light signal 642. Inanother example, a first light source may be weighted more heavily thana second light source in combining to form a combined detected lightsignal 642.

Plot 660 may include detected ambient signal 662. Detected ambientsignal 662 may include information related to the amplitude of lightreceived during an “off” or dark period of a drive pulse modulation. Forexample, detected ambient signal may include information related tolight received during “off” period 220 of FIG. 2A, as processed byambient subtractor 162 of FIG. 1, demultiplexer 154 of FIG. 1, any othersuitable processing equipment, or any combination thereof.

In some embodiments, signal noise, ringing, and/or transients may occurat or near the point in time when a sensor is attached or removed from asubject. This can be seen, for example in detected light signal 642 attime point 618. The noise may be related to, for example, changes insignal processing, changes in front end amplification, increased amountsof reflected light when the sensor is partially out of proper position,any other suitable sources of noise, or any combination thereof. In someembodiments, this noise may be identified and ignored by the system indetermining a probe-off condition.

The signals of FIG. 6 may, for example, be obtained from a foreheadpulse oximeter sensor. Other suitable sensors may be used including, forexample, a fingertip, earlobe, wrist, arm, foot, other suitable sensor,or any combination thereof. In some embodiments, different sensors mayproduce different signals, however it will be understood that thetechniques described herein may be applied to multiple sensor types withappropriate modifications. For example, a sensor such as a fingertipsensor which may rely primarily on transmitted light rather than thereflected light of a forehead sensor may result in very high detectedlight signals being received when the sensor is improperly positioned ina dark room—rather than low signal levels. Thresholds, target values,time windows, other signal processing parameters, and any combinationthereof may be adjusted based on the sensor and measurement type.

Plot 680 indicates manually controlled test conditions used to generatethe above described detected signals. Plot 680 includes environmentallight level signal 682. Environmental light level signal 682 may includeinformation related to the amount of ambient light near the sensor.Changes in environmental light level signal 682 may be indicative of thetest conditions changing. For example, the environmental light levelsignal 682 at signal level “On” may be indicative of the room lightswhere the system is located being on, while signal level “Off” may beindicative of the room lights being off. In some embodiments, theenvironmental light level signal 682 may be indicative of the control ofroom lighting, examination lights, heat sources, any other suitablesources of ambient light, or any combination thereof. It will beunderstood that the use of “On” and “Off” values for environmental lightlevel signal 682 is merely exemplary and that light levels may varycontinuously.

Plot 680 includes sensor positioning signal 684. Sensor positioningsignal 684 may include information related to the proper or desiredpositioning of a sensor, as controlled during test conditions used ingenerating the signals of plots 640 and 660. For example, changes in thelevels of sensor positioning signal 684 may be indicative of manualchanges in the sensor position during test conditions used to generatethe signals of plots 640 and 660. In some embodiments, sensorpositioning signal 684 at signal level “On” may be indicative of thesensor being properly positioned. In some embodiments, sensorpositioning signal 684 at signal level “Off” may be indicative of thesensor being improperly positioned. For example, an improperlypositioned sensor may be a probe-off condition. It will be understoodthat the use of “Off” and “On” values for sensor positioning signal 684is merely exemplary.

In time interval 602, the system may be under constant natural lightconditions from a window with a properly attached forehead sensor.Accordingly, environmental light level signal 682 is “On” and sensorpositing signal 684 is “On.” The signal levels of time interval 602 may,for example, be indicative of signals received for 30 seconds.

In some embodiments, the behavior of detected light signal 642 anddetected ambient signal 662 in time interval 602 may be indicative of abaseline behavior in the system. In time interval 602, environmentallight level signal 682 remains constant and the sensor is properlypositioned. As illustrated the signal levels of both detected lightsignal 642 and detected ambient signal 662 remain relatively constant.In some embodiments, this baseline behavior may be used to establish abaseline noise threshold, to establish alarm thresholds, to resetparameters, for any other suitable purpose, or any combination thereof.

In time interval 604, environmental light level signal 682 is varied andsensor positioning signal 684 is “On,” indicative of the sensor beingproperly positioned. For example, the system may be operated in a roomwith the room lights switched off and a window shade rapidly opened andclosed, while the sensor is properly positioned. The signal levels oftime interval 604 may, for example, be indicative of signals receivedfor 30 seconds.

In some embodiments, the behavior of detected light signal 642 anddetected ambient signal 662 in time interval 604 may be indicative of aninsensitivity to changing ambient light conditions when the sensor isproperly positioned. In time interval 604, environmental light levelsignal 682 varies between “Off” and “On.” As illustrated, the signallevels of both detected light signal 642 and detected ambient signal 662remain relatively constant, despite this variation. In some cases,stronger ambient light may result in a change in the detected lightlevels. It is expected, however, that a properly positioned sensor willreduce the impact of varying ambient light conditions on detectedsignals. That is, when the sensor is properly positioned, changes inambient light may have little effect on the detected light signal 642and the detected ambient signal 662.

In time interval 606, environmental light level signal 682 is “Off.” Forexample, the system may be operated in a dark room. In time interval606, sensor positioning signal 684 is “On,” indicative of the sensorbeing properly positioned. The signal levels of time interval 606 may,for example, be indicative of signals received for 30 seconds under darkconditions with a properly attached forehead sensor.

In some embodiments, the behavior of detected light signal 642 anddetected ambient signal 662 in time interval 606 may be indicative of abaseline behavior in a dark room of the system. In time interval 606,environmental light level signal 682 remains “Off” and the sensorpositioning signal remains at “On.” As illustrated the signal levels ofboth detected light signal 642 and detected ambient signal 662 remainrelatively constant. As illustrated, both the detected light signal 642and detected ambient signal 662 are received at substantially the sameamplitudes as in time interval 602 where the environmental light levelwas higher. This may be, for example, indicative of a properlypositioned sensor's ability to exclude ambient light effects.

In time interval 608, environmental light level signal 682 is varied.For example, the system may be operated in a room with the room lightsswitched on and off. In time interval 608, sensor positioning signal 684is “On,” indicative of the sensor being properly positioned.

In some embodiments, the behavior of detected light signal 642 anddetected ambient signal 662 in time interval 608 may be indicative of aninsensitivity to changing environmental light conditions when the sensoris properly positioned. In some embodiments, the conditions in timeinterval 608 may be similar to those in time interval 604, except alevel of electric light is varying as opposed to the amount of sunlightentering the room. As illustrated, the signal levels of both detectedlight signal 642 and detected ambient signal 662 remain relativelyconstant, despite the variations in environmental light. This mayindicate that the sensor is properly positioned.

In time interval 610, environmental light level signal 682 may be “On.”For example, the system may be operated in a room with the room lightsswitched on. In time interval 610, the sensor may be removed andreapplied. Sensor positioning signal 684 may be “On” during thebeginning of time interval 610, “Off” between time point 618 and timepoint 620, and “On” following time point 620. For example, the sensormay have been removed at time point 618 and reapplied at time point 620.

In some embodiments, the behavior of detected light signal 642 anddetected ambient signal 662 in time interval 610 may be indicative ofthe sensor being removed and reattached while operating in anilluminated room. In time interval 610, the sensor (e.g., a foreheadsensor) may be removed from the patient at time point 618. The level ofdetected light signal 642 may increase from a relatively lower level inregion 644 to a relatively higher level in region 646 substantiallycoincident with the removal of the sensor at time point 618. The levelof detected ambient signal 662 may increase from a relatively lowerlevel in region 664 to a relatively higher level in region 668substantially coincident with the removal of the sensor at time point618. The increase in both detected light signal 642 and detected ambientsignal 662 may be indicative of an increased amount of light reachingthe detector. In some embodiments, shielding or light blockingstructures may have prevented ambient light from reaching the detectorwhile it was properly positioned, but not while it is removed from thesubject. In some embodiments, the changes in signal levels may be inpart attributed to increased ambient light reaching the detector, lightfrom the emitters that is not attenuated as much as it would have beenin a properly positioned sensor arrangement reaching the detector,sensor noise, other suitable signal sources, or any combination thereof.

At time point 620, the sensor may be reattached, as indicated by sensorpositioning signal 684 changing from “Off” to “On.” As illustrated,detected light signal 642 in region 648 following time point 620 mayreturn to substantially the same level as region 644. Detected ambientsignal 662 in region 670 may return to substantially the same level asregion 664. This may, for example, be indicative of the stability of thesystem and show that the changes in the signal levels are primarilyrelated to changes in sensor positioning.

In some embodiments, the similar behavior of both detected light signal642 and detected ambient signal 662 in region 646 and region 668,respectively, may be recognized by the system as a probe-off condition.Considering an increase in both signals at the same time may beadvantageous over considering an increase in only the detected ambientsignal or only the detected light signal. In some embodiments, theincrease may be recognized by comparing one or both signal levels to athreshold. In some embodiments, signal levels, trends, slopes,derivatives, behaviors, other related signals, or combinations thereofmay be compared to a threshold, target value, other suitable criteria,or any combination thereof. For example, both the ambient and thedetected light signal crossing a threshold at substantially the sametime may be recognized by the system as a probe-off condition.

In time interval 612, environmental light level signal 682 may be “Off.”For example, the system may be operated in a dark room. In time interval612, the sensor may be removed at time point 622, as indicated by thechange in sensor positioning signal 684 at time point 622 from “On” to“Off.”

In some embodiments, the behavior of detected light signal 642 anddetected ambient signal 662 in time interval 612 may be indicative ofthe sensor being removed while operating in a dark room. In timeinterval 612, the sensor (e.g., a forehead sensor) may be removed fromthe patient at time point 622. The level of detected light signal 642may decrease to a relatively lower level in region 650 substantiallycoincident with the removal of the sensor at time point 622. The levelof detected ambient signal 662 may remain relatively constant in region672 substantially coincident with the removal of the sensor at timepoint 622. The decrease in detected light signal 642 coincident with theconstant level of detected ambient signal 662 may be indicative of lesslight from the emitters reaching the detector. In a reflectivearrangement such as a forehead sensor, this would be expected when thesensor is removed because it relies on a reflective structure for lightto be communicated from the emitters to the detectors. The relativelyconstant level of detected ambient signal 662 in time interval 612 maybe indicative of a relatively constant amount of light reaching thedetector when the environmental light level signal 682 is at “Off.”

In time interval 614, the system may be operated in a room with the roomlights switched on and the sensor reattached at time point 624. Theenvironmental light level signal 682 is “On.” At time point 624, sensorpositioning signal 684 may change from “Off” to “On.”

For example, the behavior of detected light signal 642 and detectedambient signal 662 in time interval 614 may be indicative of a lightsource being switched on followed by a sensor being reattached to apatient. The level of detected light signal 642 may increase from therelatively lower level in region 650 to a relatively higher level inregion 652 substantially coincident with the increase in environmentallight. The level of detected light signal 642 may decrease in region 654substantially coincident with the reattachment of the sensor at timepoint 624. The level of detected ambient signal 662 may increase fromthe relatively lower level in region 672 to a relatively higher level inregion 674 substantially coincident with the increase in environmentallight. The level of detected ambient signal 662 may decrease in region676 substantially coincident with the reattachment of the sensor at timepoint 624.

In some embodiments, the behavior in time intervals 612 and 614 mayillustrate the benefit of using a comparison of detected light signal642 and detected ambient signal 662, rather than a single signal, todetect a probe-off condition. For example, there is no significantchange in detected ambient signal 662 at time point 622, due to the darkroom, despite the sensor being removed as indicated by sensorpositioning signal 684. Using only detected ambient signal 662 wouldresult in a false-negative, since the probe-off condition would not bedetermined. However, the probe-off condition at time point 622 can bedetermined by also using the detected light signal 642. By monitoringboth detected light signal 642 and detected ambient signal 662, thesystem may detect conditions such as mimicking-equal,mimicking-parallel, or nonlinear scaling behavior, as described above.The system may identify these behaviors as being indicative of aprobe-off condition.

It will be understood that the system may identify the change using anysuitable signal levels, trends, or any combination thereof, along withany suitable thresholds, targets, other suitable criteria, or anycombination thereof. Any suitable polling, averaging, index values, orother suitable combination of multiple criteria or tests may be used.For example, the system may require a particular number of thresholdcrossings among one or more signals to indicate a probe-off condition.

In time interval 616, the system may be operated in a room with the roomlights switched on while the sensor is removed from the subject and, forexample, placed on a surface with the detector receiving light both fromthe emitters and from environmental sources. In this position, thesystem receives the same amount of ambient light during both the “on”and “off” periods of the emitted photonic signal, and receives aconstant emitted photonic signal component during the “on” period due tolight from the emitters reaching the detector directly. Accordingly,there is a parallel, offset behavior between detected light signal 642and detected ambient signal 662 that will be shown in further detailbelow in time interval 740 of FIG. 7. The environmental light levelsignal 682 remains “On” in time interval 616, and the sensor positioningsignal 684 changes from “On” to “Off” at time point 626.

In some embodiments, the behavior of detected light signal 642 anddetected ambient signal 662 in time interval 616 may be indicative ofsignal behavior when a forehead sensor is positioned away from a subjectin the presence of ambient light. At time point 626, the sensor may beremoved from the subject. In region 656, detected light signal 642 maydecrease to a relatively lower level as compared to the level beforetime point 626. This may be indicative of a decreased amount of lightreaching the detector due to the lack emitted light reflected by apatient, yet some emitted light may still reach the detector due todiffusion and reflection from other surfaces. In region 678, detectedambient signal 662 may increase to a relatively higher level as comparedto the level before time point 626 due to increased ambient lightreaching the now-exposed detector. The system may recognize elements ofthis behavior as relating to a probe-off condition, as will be describedin further detail below.

FIG. 7 shows illustrative plots 740 and 780 of physiological monitoringsystem signals in accordance with some embodiments of the presentdisclosure. The plots of FIG. 7 may include information from the plotsof FIG. 6 shown in more detail. Plot 740 may include information fromtime intervals 610, 612, 614 and 616 of both plot 640 and plot 660 ofFIG. 6. For example, time intervals 710, 712, 714, and 716 may relate totime intervals 610, 612, 614 and 616 of FIG. 6, respectively. Similarly,time points 718, 720, 722, 724, and 726 may relate to time points 618,620, 622, 624, and 626 of FIG. 6, respectively.

Plot 740 and plot 780 have abscissa axes in units of time. In someembodiments, the axes may be on the same time scale. For example, theplots as illustrated may show approximately three minutes of signals.Plot 740 may have ordinate axes in units of amplitude. As illustrated,plot 740 may include information from both plot 640 and plot 660 of FIG.6 on a shared ordinate axis. Plot 780 may have arbitrary ordinate axes.

Plot 740 may include detected light signal 742 and ambient light signal762. The amplitude of the detected light signal 742 and ambient lightsignal 762 may be indicative of the amount of light detected by adetector during an “On” period and “Off” period, respectively, of alight drive modulation. The amplitude of the detected light signal 742may include an IR, red, other suitable light signal, or any combinationthereof. As depicted, detected light signal 742 also includes ambientlight.

Characteristics of plot 740 may relate to the characteristics describedfor plots 640 and 660 of FIG. 6. Plot 780 may include light level signal782 and sensor positioning signal 784. Characteristics of plot 780 mayrelate to the characteristics indicated by plot 680 of FIG. 6. Thesignals may be obtained and processed as described above for the signalsillustrated in the plots of FIG. 6.

In time interval 710, the sensor may be removed at time point 718 andreattached at time point 720, as indicated by sensor positioning signal784. The sensor may be operating in an illuminated environment, asindicated by environmental light level signal 782. In region 744, thelevel of detected light signal 742 may be relatively higher than thelevel of detected ambient signal 762 because, for example, the sensor isproperly positioned and some or all of the ambient light is blocked fromreaching the detector by the sensor body. In region 746 following timepoint 718 where the sensor is removed from the subject and directlyexposed to environmental light, detected light signal 742 and detectedambient signal 762 may go to a relatively higher, common level. Thiscommon movement may be referred to as mimicking-equal behavior, and maybe recognized as being indicative of a probe-off condition. The behaviormay be identified, for example, by subtracting detected light signal 742from ambient detected light signal 762 and generating a substantiallyzero amplitude signal within a time window. A certain amount ofdeviation from zero may be permitted in identifying the mimicking-equalbehavior. Deviations may arise from line noise, equipment noise, anyother source, or any combination thereof. In some embodiments, thesignals in region 746 may be indicative of mimicking-parallel behavior.Following time point 720, where the sensor is reattached as indicated bysensor positioning signal 784, the levels of both detected light signal742 and detected ambient signal 762 return to levels similar to those inregion 744 prior to time point 718. In some embodiments, this behaviormay not be recognized if only changes in detected light signal 742 wereanalyzed to detect a probe-off condition. The movement of both signalsto a common, higher level with common movement indicates that both thedetected emitted signal and the detected ambient signal are comprisedsubstantially of environmental light. For example, the sensor receivingno, or a small amount of, light corresponding to the emitted photonicsignals. The system may identify this behavior as indicating that thesensor is not properly positioned on the patient.

In time interval 712, the sensor may be operating in a dark environment,as indicated by environmental light level signal 782. At time point 722,the sensor may be removed from the subject. In region 748, the level ofdetected light signal 742 may be relatively higher than the level ofdetected ambient signal 762 due to the reflected emitted light reachingthe detector and the lack of environmental light. In region 750following time point 722, where the sensor is removed from the subject,the level of detected light signal 742 may drop to a level similar tothe level of detected ambient signal 762. The level of detected ambientsignal 762 may remain relatively unchanged, due to the lack ofenvironmental light. This similarity between detected light signal 742and detected ambient signal 762 in region 750, sometimes referred to asmimicking-equal, or following behavior, may be recognized as a probe-offcondition. In some embodiments, mimicking-equal in combination with alow detected light signal amplitude may further be recognized as beingindicative of a probe-off condition.

In time interval 714, the sensor may operate in an illuminatedenvironment, as indicated by environmental light level signal 782. Thesensor may be removed from the patient during region 752 of timeinterval 714, reattached at time point 724, and remain attached duringregion 754. In region 752, where the sensor is operating removed fromthe subject in an illuminated room, changes in detected light signal 742and detected ambient signal 762 may parallel each other. In someembodiments, the system may identify this behavior as being indicativeof a probe-off condition, as described above. In some embodiments,region 752 may display a relatively higher amount of signal noise than,for example, region 750. In some embodiments, signal noise may berelated to an improperly positioned sensor being moved in relation toambient light sources, to cable connector noise, to other sources ofnoise, or any combination thereof. In some embodiments, the system mayidentify the increased noise as being indicative of a probe-offcondition. In region 754 after time point 724, where the sensor isreattached as indicated by sensor positioning signal 784, detected lightsignal 742 and detected ambient signal 762 may return to varyingindependently.

In time interval 716, the sensor may be removed from the patient andplaced, for example, on a table. The detector may be oriented such thatit receives environmental light and a small amount of the emittedphotonic signal, for example, by reflection from a surface. The sensormay be removed from the subject at time point 726 coinciding with thestart of time interval 716. Behavior in time interval 716 may beindicative of a probe-off condition and will be discussed in detail inrelation to FIG. 8 below. In some embodiments, a small offset may bepresent between detected light signal 742 and detected ambient signal762. The offset may be, for example, due to the presence of somereflected emitted light, in contrast to the common movement of region746 where none of the emitted photonic signal reaches the detector. Thebehavior may be referred to as mimicking-parallel and will be shown indetail in FIG. 8 below.

FIG. 8 shows illustrative plot 800 of physiological monitoring systemsignals in accordance with some embodiments of the present disclosure.FIG. 8 may include information from the plots of FIG. 6 and FIG. 7 shownin more detail. Plot 800 may include information from time interval 616of both plot 640 and plot 660 of FIG. 6. Plot 800 may includeinformation from time interval 716 of FIG. 7 in more detail.

Plot 800 may include detected light signal 842 and ambient light signal862. The amplitude of the detected light signal 842 and detected ambientsignal 862 may be indicative of the amount of light detected by adetector during an “On” period and “Off” period, respectively, of alight drive modulation. The amplitude of the detected light signal 842may include an IR, red, other suitable light signal, or any combinationthereof. As depicted, detected light signal 842 also includes ambientlight.

Characteristics of plot 800 may relate to the characteristics describedfor plots 640 and 660 of FIG. 6. Plot 880 may include light level signal882 and sensor positioning signal 884. Characteristics of plot 880 mayrelate to the characteristics indicated by plot 680 of FIG. 6.

The signals in plot 800 may include information from a sensor operatingin an illuminated room, as indicated by environmental light level signal882. The sensor, for example a forehead sensor, may be attached in timeinterval 802 and removed at a particular time, as indicated by timepoint 886 of sensor positioning signal 884. Upon removal of the sensor,it may be seen in time interval 804 that detected ambient signal 862 anddetected light signal 842 vary together with a small, parallel offset.In the time immediately following removal of the sensor, there may be anincreased amount of noise present in both signals. The system mayrecognize this increased amount of noise as being indicative of aprobe-off condition. Additionally, the mimicking-equal behavior of timeinterval 804 which may correspond to an equal or approximately equalamount of light reaching the detector during both detected light signaland detected ambient signal periods, may be recognized by the system asbeing indicative of a probe-off condition.

In some embodiments, the mimicking-parallel behavior of the signals inregion 806 may be indicative of a situation where the probe is removedand placed on or near a moderately reflective surface at time point 808.In an example, the emitter and detector may be facing the surface suchthat some of the emitted light reaches the detector and the sensor bodyblocks a portion of the ambient light from reaching the sensor. Offset810 may be indicative of a particular amount of light from the emittersreaching the detector during the drive pulse cycle. Since this light isinteracting with a constant, unchanging surface (as compared to subjecttissue), the amount of reflected light is constant. Thus, the variationsin the signals are attributed to variations in the ambient light, andthe offset is attributed to the emitters. This relatively small offsetwith parallel signal behavior may be recognized by the system as beingindicative of a probe off situation.

In some embodiments, offset 810 may be determined by subtractingdetected ambient signal 862 from detected light signal 842. Thissubtraction would, for the illustrated signals, result in a small,relatively constant signal. This signal, or variations in the signal,may be compared to a threshold or target value to identify a probe-offcondition. A threshold may be defined at a predetermined value, by userinput, by historical information, by information related to theequipment, by any other suitable criteria, or any combination thereof.For example, the threshold may allow parallel behavior to be recognizedwithin a tolerance level, such that signals that display a certaindegree of parallel behavior are identified as indicating a probe-offcondition. Variations in the offset or any other signal may be analyzedusing, for example, slope, fractal dimension, any other suitabletechnique, or any combination thereof. In some embodiments, the systemmay use more complex mathematical comparisons of the two or moresignals, as described above.

In some embodiments, offset parallel behavior may be observed by thesystem due to non-linear system response or processing. For example,sensor signal amplitude may be greater with respect to the number ofincident photons at low light levels as compared to high light levelsdue to detector saturation. Offsets and non-linearity may also occur asa result of front end gain, filters, emitter spectrum variations,ambient light spectrum variations, other suitable sources of variations,or any combination thereof.

Non-linearity in the system may result in similar behavior between twosignals where a relative change is apparent between the two signals. Forexample, where a detected ambient signal level increases by 100 mV thendecreases by 200 mV, the detected light level may signal may increase 50mV and then decrease by 100 mV. In some embodiments, the system mayremove non-linearity by suitable processing steps, may detect probe-offconditions in the presence of non-linear comparisons by, for example,comparing the slopes of signals, may consider non-linearity by any othersuitable technique, or any combination thereof.

FIG. 9 is flow diagram 900 showing illustrative steps for determining aprobe-off condition in accordance with some embodiments of the presentdisclosure.

In step 902, the system may use the physiological sensor to emit aphotonic signal. The system may emit a photonic signal including onewavelength of light, multiple wavelengths of light, a broad spectrumlight (e.g., white light), or any combination thereof. For example, thephotonic signal may include light from a red LED and light from an IRLED. The emitted photonic signal may be emitted, for example, by lightsource 130 of FIG. 1. In some embodiments, the emitted photonic signalmay include a light drive modulation. For example, where the photonicsignal includes a red light source and an IR light source, the lightdrive modulation may include a red drive pulse followed by an “off”period followed by an IR drive pulse followed by an off period. It willbe understood that this drive cycle modulation is merely exemplary andthat any suitable drive cycle modulation or combination of modulationsmay be used. In some embodiments, the photonic signal may include acardiac cycle modulation, where the brightness, duty cycle, or otherparameters of one or more emitters are varied at a rate substantiallyrelated to the cardiac cycle.

In step 904, the system may receive a light signal. The received lightsignal may include light from drive pulses or other emitted light in theemitted photonic signal that has interacted with the subject. Thereceived light signal may be detected by, for example, detector 140 ofFIG. 1. In some embodiments, a portion of the emitted light may bepartially attenuated by the tissue of the subject before being receivedas a received light signal. In some embodiments, the received light mayhave been primarily reflected by the subject. For example, reflectedlight may be detected by a forehead-attached system where the emitterand detector are on the same side of the subject. In some embodiments,the received light may have been transmitted through the subject. Forexample, transmitted light may be detected in a fingertip-attached orearlobe-attached sensor.

In some embodiments, the received light signal may include an ambientlight signal component and a component related to the emitted photonicsignal. As described above, the system may determine a detected ambientsignal, any other suitable signals, or any combination thereof.

In some embodiments, the detected ambient signal may, for example,include ambient signal 222 of FIG. 2B. In some embodiments, the systemmay subtract ambient signal 222 or a signal derived from ambient signal222 from the received signal to generate an adjusted signal. Theadjusted signal may be used to determine physiological parameters. Insome embodiments, the system may determine an ambient signal forprobe-off analysis before generating the adjusted signal. Separation ofthe ambient signal from the received signal may include, for example,demultiplexer 154 of FIG. 1, any other suitable equipment, or anycombination thereof. Signal processing of the ambient component andemitted light component may include any suitable components ofphysiological monitoring system 100 of FIG. 1, physiological monitoringsystem 310 of FIG. 3, signal processing system 400 of FIG. 4, any othersuitable components, or any combination thereof.

In some embodiments, the system may adjust or compensate a signaldepending in part on the LED drive signal, the detector gain, othersuitable system parameters, or any combination thereof. For example,increasing the gain on a detected signal may result in an increasedambient signal. The system may compensate for this increased ambientthat is not correlated with a change in the sensor positioning. In afurther example, the system may change the LED emitter brightness,resulting in a change in the detected signals. The system may compensatefor these changes in the detected signal amplitude to distinguish themfrom a change in the sensor positioning. It will be understood that thesystem may make any adjustments in gain, amplification, frequency,wavelength, amplitude, any other suitable adjustments, or anycombination thereof. It will be understood that the adjustments may bemade to the emitted photonic signal, the received signal, a signalfollowing a number of processing steps, any other suitable signals, orany combination thereof.

In step 906, the system may identify a substantially constant amplitudein the light signal. This behavior may be referred to as flatness. Thelight signal may include, as described above, a drive pulse lightsignal, an ambient light signal, any suitable processed signal, or anycombination thereof. A substantially constant amplitude may beidentified by a flatness value determined by comparing the signal to atarget value, by analyzing a derivative of the signal, by determining astatistical parameter, determining a slope, determining a fractal orother dimension, by any other suitable technique, or any combinationthereof. In some embodiments, a flatness value is compared to athreshold. For example, a moving average of a first derivativesubstantially close to zero may be recognized as a substantiallyconstant amplitude. In another example, a slope between two particularthreshold amounts may be recognized as a substantially constantamplitude. Characteristics and parameters may be considered, forexample, within a fixed or moving time window. Statistical parametersmay include means, standard deviations, variances, averages, filteredvalues, trends, any other suitable parameters, or any combinationthereof. In some embodiments, a target value or other characteristicevaluation parameter may be based on system parameters, physiologicalcharacteristics, historical information, user input, any other suitableparameters, or any combination thereof. For example, a certainconfiguration of probe elements may be known to generate a particularsignal level under standard ambient light levels when detached from asubject. In some embodiments, a signal level may be compared to a targetor threshold by any suitable technique. For example, a signal level maybe subtracted from a target level to identify a substantially constantamplitude.

In step 908, the system may determine the position of the physiologicalsensor based in part on the identification of the constant amplitude.For example, the system may determine a probe-off condition when asubstantially constant amplitude is identified in one or more receivedlight signals, or based on a combination of signals. For example, thesystem may determine the presence of a substantially constant amplitudein a detected light signal, an ambient light signal, any other suitablesignal, or any combination thereof, and that constant amplitude may berecognized by the system as being indicative of a probe-off condition.It will be understood that a substantially constant amplitude alone maynot necessarily indicate a probe-off condition. For example, flatness ofthe detected ambient signal may be a normal condition. In someembodiments, the system may consider a flatness in the detected lightsignal to be indicative of a probe-off condition. In some embodiments,the system may consider both the detected ambient signal and thedetected light signal in determining a probe-off condition. For example,the system may consider a substantially constant amplitude of thedetected light signal in combination with a high level of the detectedambient signal to be indicative of a probe-off condition, for example,as shown in region 806 of FIG. 8. In some embodiments, the constantamplitude may be related to a maximum detector input voltage or current.For example, an analog to digital converter (or other front endprocessing circuitry) may receive the maximum value it can process whena probe-off condition occurs in a fingertip sensor. In some embodiments,a substantially constant amplitude may be related to a minimum detectorinput, for example, when the light signal amplitude is substantiallyclose to zero.

In an optional step (not shown), an indication of a probe-off conditionmay be provided as described above in relation to flow diagram 500 ofFIG. 5.

FIG. 10 shows illustrative plot 1000 of a physiological monitoringsystem signal in accordance with some embodiments of the presentdisclosure. Signal 1002 may include any suitable system signal. Forexample, signal 1002 may include a detected light signal, an ambientlight signal, a combination of detected light signals, any othersuitable signals, or any combination thereof. In some embodiments,signal 1002 may be the signal received in step 904 of FIG. 9. In timeinterval 1004, a sensor of the system may be properly positioned and thesystem may be operating normally. The signal may show variations inamplitude related to physiological and/or system parameters. In timeinterval 1008 following time point 1006, signal 1002 may display asubstantially constant amplitude. Time point 1006 may be related to apoint where the system enters a probe-off condition. The system mayidentify the substantially constant amplitude or flatness of signal 1002in time interval 1008. For example, the system may identify thesubstantially constant amplitude using the techniques described in step906 of FIG. 9. The system may identify this probe-off condition based onthe substantially constant amplitude of signal 1002 in time interval1008 using the techniques described in step 908 of FIG. 9.

The foregoing is merely illustrative of the principles of thisdisclosure and various modifications may be made by those skilled in theart without departing from the scope of this disclosure. The abovedescribed embodiments are presented for purposes of illustration and notof limitation. The present disclosure also can take many forms otherthan those explicitly described herein. Accordingly, it is emphasizedthat this disclosure is not limited to the explicitly disclosed methods,systems, and apparatuses, but is intended to include variations to andmodifications thereof, which are within the spirit of the followingclaims.

What is claimed:
 1. A method for determining whether a physiologicalsensor is properly positioned on a subject, the method comprising:receiving a light signal using the physiological sensor; processing,using processing equipment, the light signal to obtain a first signalcorresponding to ambient light; processing, using the processingequipment, the light signal to obtain a second signal corresponding toan emitted photonic signal and ambient light; analyzing, using theprocessing equipment, the first signal and the second signal to identifysimilar signal amplitude behavior; and determining, using the processingequipment, that the physiological sensor is not properly positionedbased on the analysis.
 2. The method of claim 1, wherein thephysiological sensor comprises a pulse oximeter.
 3. The method of claim1, wherein receiving the light signal comprises receiving light from afirst light emitting diode configured to emit a first wavelength oflight and from a second light emitting diode configured to emit a secondwavelength of light.
 4. The method of claim 1, wherein the light signalis detected by a photoelectric detector.
 5. The method of claim 1,wherein analyzing the first signal and the second signal to identifysimilar signal amplitude behavior comprises determining whether adifference in amplitudes of the first and second signals issubstantially constant.
 6. The method of claim 1, wherein analyzing thefirst signal and the second signal to identify similar signal amplitudebehavior comprises determining whether a difference in slopes of thefirst and second signals is substantially constant.
 7. The method ofclaim 1, wherein analyzing the first signal and the second signal toidentify similar signal amplitude behavior comprises compensating fornonlinearity.
 8. The method of claim 1, wherein analyzing the firstsignal and the second signal to identify similar signal amplitudebehavior comprises: determining a value based on the first signal andthe second signal; and comparing the value to a threshold.
 9. The methodof claim 8, wherein the value is selected from the group consisting ofdifference between the first and second signal, a flatness value, aconfidence value, a slope value, an amplitude value, and combinationsthereof.
 10. The method of claim 1, wherein analyzing the first signaland the second signal to identify similar signal amplitude behaviorcomprises identifying a substantially constant amplitude in at least oneof the first signal and the second signal.
 11. The method of claim 1,wherein analyzing the first signal and the second signal to identifysimilar signal amplitude behavior comprises identifying a thresholdcrossing by both signals at substantially the same time.
 12. The methodof claim 1, wherein analyzing the first signal and the second signal toidentify similar signal amplitude behavior comprises identifying atleast one of a mimicking-equal behavior, a mimicking-parallel behavior,or a nonlinear scaling behavior of the first and second signals.
 13. Themethod of claim 1, further comprising providing an indicator that thephysiological sensor is not properly positioned.
 14. The method of claim1, further comprising compensating the received light signal for a lightdrive setting.
 15. A system for determining whether a physiologicalsensor is properly positioned on a subject, the system comprising:processing equipment configured to: receive a light signal from thephysiological sensor; process the light signal to obtain a first signalcorresponding to ambient light; process the light signal to obtain asecond signal corresponding to an emitted photonic signal and ambientlight; analyze the first signal and the second signal to identifysimilar signal amplitude behavior; and determine that the physiologicalsensor is not properly positioned based on the analysis.
 16. The systemof claim 15, wherein the processing equipment comprises a pulseoximeter.
 17. The system of claim 15, wherein the processing equipmentis further configured to determine whether a difference in amplitudes ofthe first and second signals is substantially constant.
 18. The systemof claim 15, wherein the processing equipment is further configured todetermine whether a difference in slopes of the first and second signalsis substantially constant.
 19. The system of claim 15, wherein theprocessing equipment is further configured to: determine a value basedon the first signal and the second signal; and compare the value to athreshold.
 20. The system of claim 15, wherein the processing equipmentis further configured to identify a substantially constant amplitude inat least one of the first signal and the second signal.
 21. The systemof claim 15, wherein the processing equipment is further configured toprovide an indicator that the physiological sensor is not properlypositioned.
 22. The system of claim 15, wherein the processing equipmentconfigured to analyze the first signal and the second signal to identifysimilar signal amplitude behavior comprises processing equipmentconfigured to identify at least one of a mimicking-equal behavior, amimicking-parallel behavior, or a nonlinear scaling behavior of thefirst and second signals.
 23. The method of claim 1, wherein the ambientlight corresponds to light received from light sources other than lightsources coupled to the physiological sensor.
 24. The system of claim 15,wherein the ambient light corresponds to light received from lightsources other than light sources coupled to the physiological sensor.